An increasing number of information services are now offered via satellite communication. Specifically, there are a wide variety of satellites positioned in geostationary orbit about the earth for providing various services to users on the ground. Such services include, for example, one-way (also referred to as receive-only) services such as television services, and two-way (also referred to as transmit and receive) services such as Internet communications. Unfortunately, many related services are offered on different satellites. For example, general satellite TV programming may be provided on one satellite, while Internet services are offered by another satellite, while still other satellites may offer high definition TV programming or foreign language TV programming. A user that subscribes to two or more of these services must have the ability to communicate with each of the satellites that provide the selected services. While this can be accomplished using different antennas corresponding to each satellite, this solution is neither practical nor acceptable for most customers. For this reason, there has been a significant uptake in the past few years in technology that allows for use of one antenna solution to communicate with multiple satellites. These antenna solutions are sometimes referred as multi-beam antennas.
There are various issues associated with the design of multi-beam antennas. One such issue is reflector profile. Specifically, multi-beam antennas include various feeds for communication with different satellites. For example, if an antenna is designed to communicate with three separate satellites, the antenna will include three separate feeds, one associated with each of the satellites. These feeds are all spaced in front of the reflector of the antenna. For proper communication, the feeds must be oriented properly with respect with the reflector in order to optimize reception and/or transmission of signals between the feed and its associated satellite, while avoiding crosstalk with other satellites. In a multi-beam solution, a reflector having an elliptical profile is generally preferred over a reflector having a circular profile. Specifically, a circular reflector generally does not narrow the beam of the signals received from the satellites. As such, all of the beams overlap significantly near the focal point of the reflector. A reflector having an elliptical profile, on the other hand, can be configured such that signals from different satellites can be generally focused at different points in front of the reflector. Specifically, it is necessary to make the beams transmitted by the satellites narrower in the azimuth plane (i.e., along the geostationary arc) to avoid interference or crosstalk from the closely adjacent satellites. Consequently, it is necessary to employ an antenna having a profile that is narrower in the vertical direction than in the horizontal direction; such as for example, an elliptical reflector, rectangular, or similar non-circular profile, (i.e., one having an aspect ratio of greater than one).
While use of a reflector having an elliptical profile allows multiple feeds to be placed in the same antenna solution, there are some drawbacks to use of these reflectors. Specifically, some satellites communicate via circularly polarized signals, as opposed to linear polarized signals. A circularly polarized signal consists of two vector components that are rotationally oriented ninety (90) degrees relative to each other. Further, the vector components have the same magnitude. To maintain the integrity of the signal, the vectors must remain substantially at the same magnitude, and they must remain substantially orthogonal to each other. To maintain the integrity of a circularly polarized signal, the vectors must remain substantially at the same magnitude, and they must remain substantially orthogonal to each other. Circular antenna reflectors maintain this electrical symmetry. Elliptical reflectors, on the other hand, do not maintain this symmetry because of their different dimensions in the horizontal and vertical directions. Thus, elliptical reflectors are typically not used with circular polarized communications, thereby making it difficult to provide a multi-beam solution where at least one of the satellites communicates using circularly polarized signals.
An additional issue with multi-beam solutions is satellite position spacing. As more satellites are introduced into orbit, the angular spacing between the satellites will decrease. In fact, currently there are several satellites that are positioned within a range of 5 degrees or less of arc with respect to each other. The proximity of these satellites to each other is somewhat problematic from the standpoint of using one antenna to establish individual communication links with both of these satellites.
Specifically, to communicate with multiple satellites, an antenna will typically include individual feeds dedicated to communicating with one of the satellites. Because of the closeness in angular proximity of some satellites, these wave-guides should be placed in close proximity to each other on the antenna to properly communicate with their respective satellites. The problem is that many conventional corrugated wave-guide designs cannot be used, because of the reduced spacing required between the phase centers of the wave-guides needed to receive from and transmit signals to the satellites is such that the conventional individual wave-guides would occupy overlapping space due to their size.
In view of these concerns, applicant has created various multi-beam antenna solutions for specific communication environments. For example, U.S. Pat. No. 6,480,165, entitled “Multi-beam Antenna For Establishing Individual Communication Links With Satellites Positioned In Close Angular Proximity To Each Other” discloses a multi-beam antenna solution for communicating with closely spaced satellites, where one feed is configured for two-way communication and the other feed is configured for one-way communication. In this antenna solution, one of the feeds is filled with a dielectric material. The use of the dielectric material allows the feed to made smaller in size, which in turn, allows the two feeds to be spaced in close proximity for communicating with the closely spaced satellites.
Recently, applicant also developed a feed solution that allows communication of circular polarized signals using a reflector having a non-circular profile, such as an elliptical profile. Specifically, U.S. patent application Ser. No. 10/370,166 filed Feb. 20, 2003 and entitled “Circularly Polarized Receive/Transmit Elliptic Feed Horn Assembly For Satellite Communications” discloses a feed solution formed of a plurality of corrugations. The corrugations progressively transition from substantially circular at an end closest to the receiver, to substantially non-circular at the opposite end of the corrugated section that faces the reflector. The non-circular corrugations are configured to correct the distortions of the circularly polarized signals induced by the non-circular reflector profile.
The above developments address many of the issues relating to multi-beam antenna solutions. Specifically, these systems provide solutions for communicating with closely spaced satellites and communication using circularly polarized signals in an antenna solution that uses a reflector having an elliptic profile. However, there are other issues yet to be addressed. Specifically, there is currently a need for an antenna solution that facilitates communication with at least two closely spaced satellites, where one of the feeds is capable of being configured to either communication circularly polarized or linearly polarized signals based on its configuration.
This antenna solution involves communication with at least one Fixed Satellite Services (FSS). FSS is a two-communication system (i.e., both transmit and receive) for internet, data, voice, etc. communications. FSS has somewhat more stringent standards than the more traditional direct Broadcast Satellite Services (BSS). Specifically, FSS has a more stringent rejection standard for closely spaced feeds. The communication beam must be narrower in the azimuth plane to avoid interference. FSS requires at least a 12 dB drop off. This minimum drop off ensures that there is not an excess level of crosstalk between adjacent feeds.
An added issue with multi-beam antenna solutions is the transition from use of satellites that communicate using linear polarization to satellites that communicate using circular polarized signals, or visa versa. Specifically, there are current antenna solutions that communicate with satellites that use linear polarization for communication. Plans are to replace some of these satellites with satellites that communicate using circularly polarized signals. As such, prior to replacement of the satellite, an antenna solution is needed that communicates using linearly polarized signals. However, after replacement, an antenna solution is needed that communicates using circularly polarized signals. One solution would to retrofit each antenna when the transition occurs. This, unfortunately, is not a viable solution.
On the one hand, satellite spacing requirements demand an elliptic aperture to eliminate cross talk and to provide higher level of signal isolation at two degree adjacency. On the other hand, both direct broadcast satellite (DBS) and future FSS satellites are typically designed to operate with circularly polarized signals, either Right Handed or Left Handed (RHCP/LHCP) ground antennas. Consequently, the reflector and feed horn assemblies should be versatile to accommodate the two degree satellite rejection and at the same time, operate in both linearly and/or circularly polarized environment. The combined solution of multi-satellite operation, cross talk, and circularly polarized requirements is an elliptical reflector profile that establishes satellite communications link and functions in both linearly and circularly polarized environments. However, the reflector ellipticity destroys the system symmetry as required for circularly polarization and creates a high level of axial ratio or cross-polarization.
In light of the above, a new antenna solution is needed that allows a multi-beam antenna to communicate with two closely spaced satellites, where both satellites use one-way communication, at least one of the satellites is an FSS satellite, and where one of the feeds is capable of being configured to communicate either linear polarized signals or circularly polarized signals.